This invention relates to a unique non-invasive liquid level detection apparatus, applied to vessels, for example steam boilers, expansion tanks for hot water hydronic systems, and blow down tanks, that incorporate a gauge glass for visual indication of liquid level within the vessel.
Liquid level sensing for steam boilers is critically important because operation of a boiler with less than the minimum required volume of water can result in catastrophic explosions and fires that can be lethal to the public and destructive to property and equipment. It is for this reason that the American Society of Mechanical Engineers Code (ASME CSD-1) and the American National Standards Institute Code (ANSI Z21.13a) mandate that every steam boiler include both a visual gauge glass for verification of proper water level within the boiler, and a safety control known as a Low Water Cutoff for automatically turning off the energy source to the boiler if the water level falls below the minimum safe level as specified by the manufacturer of the boiler.
There are two major types of liquid level sensors available for the protection of steam boilers. Those two types are mechanical float-actuated sensors, and electronic probe sensors. Each involves undesirable invasion of the interior boiler space for liquid level detection. Following is a discussion of each sensor type.
For decades, liquid level sensing for steam boilers has been provided by a mechanical apparatus consisting of a bulky bolted-on cast-iron float chamber attached to the boiler at the water line and communicating with the inside volume of the boiler. Float-actuated switches within the bolted-on float chamber are utilized to turn off the energy source for the boiler if the water level in the boiler should fall below a minimum acceptable level. Typically, electrical switches are also provided to open an electronic water-feed valve to restore the proper water level within the boiler. Deficiencies of mechanical liquid level detection systems include:
Mechanical float-actuated liquid level sensors as applied to a boiler require a relatively large float chamber, typically constructed of cast iron, that must be bolted and sealed to the boiler so that boiler water can enter the chamber under conditions of high temperature and pressure without leaking outside the boiler. Leakage and gasket repair are common problems in the industry.
A float chamber functions as a water reservoir, and boiler sediment, scale, minerals, and salts tend to be deposited within the chamber. A laborious daily procedure called “boiler blowdown” is required for steam boilers to drain water and scale from the float chamber. A poorly maintained float chamber collects scale and sediment and may become partially or even completely filled with sediment such that the float cannot operate, thereby compromising the code-mandated low-water protection for the boiler. Silted up float chambers are also a common problem in the industry.
Mechanical float-actuated liquid level sensors incorporate multiple intricate moving parts including floats, float arms, float-arm pivots, and switch contacts that are subject to wear and breakage, or to fouling and “hanging up” due to sedimentation.
Mechanical float-actuated liquid level sensors are relatively expensive and bulky as compared to electronic controls. Float-actuated controls are also difficult to install and to maintain, prone to develop leaks at the interface with the boiler or within the air-filled float itself, and provide only a limited operating range between “low” and “high” level detection as dictated by the limited length of the float arm and the limited size of the float chamber. These controls, moreover, must be installed at a precise location on the boiler, i.e., at the boiler waterline, and are subject to field failure with the possibility of catastrophic damage to the boiler and serious injury to the public.
The industry has been looking for a suitable alternative to mechanical float-actuated liquid level sensors for years. With the advent of new electronic technologies, float-actuated liquid level sensors are declining in popularity within the boiler industry and elsewhere.
Another liquid level sensing apparatus in wide usage incorporates an electronic control typically comprising at least one probe inserted into the boiler to sense the presence or absence of water at the probe tip or along the length of the probe. These probes are typically either bare metal stainless steel probes or completely jacketed stainless steel probes with an electrically non-conducting coating applied over the entire length of the probe. Electronic liquid level sensing devices operating upon the electrical conductance between the probe and the process liquid typically utilize bare-metal probes, whereas electronic liquid-level sensing devices operating upon the electrical capacitance between the probe and the process liquid use jacketed electrically insulated probes. The purpose of the jacket is to completely isolate the bare metal of the probe from the process liquid. Capacitance sensors experience total failure if liquid touches the bare metal of the probe through a leak path in the protective jacket.
The conductance and capacitance electronic probe liquid level sensors currently in widespread use provide improvements over the mechanical float-actuated sensors, but still have significant limitations as detailed below.
Electronic probe liquid level sensors require insertion of one or more probes into the boiler or into a liquid-filled chamber attached to and communicating with the interior space of the boiler. This requirement presents mechanical sealing difficulties because of the high temperatures and pressures within a boiler.
The corrosive action of high pressure steam on the probe jacket for capacitance-type sensors can cause degradation and failure of the insulating jacket and failure of the unit.
Bare-metal conductance-type probes mounted internal to the boiler are subject to fouling with scale that can reduce the detection sensitivity of the sensor. In some cases, the level sensor may become locked in an “unsafe” mode where loss of water cannot be detected because the probe is coated with a wet paste that remains on the probe even after the water level has fallen below the level of the probe. Damage or destruction of the boiler can result in such a circumstance.
Jacketed capacitance-type probes mounted internal to the boiler may develop pin-hole leaks in the electrically insulating jacket due to the effects of high pressure and high temperature steam, with consequent immediate failure of the device.
Probes mounted internal to the boiler cannot be inspected, serviced, or replaced without taking the boiler out of service.
Electronic probe liquid level sensors with probes mounted internal to the boiler cannot be conveniently tested to verify proper operation of the liquid level sensor. Although most electronic liquid level sensors incorporate a “test” function that verifies the proper working condition of the electronic circuitry, there is no test to evaluate the integrity of the probe. The circuit test does not verify the proper working condition of the liquid level sensor taken as a whole.
Other level sensing technologies exist, but are not in wide usage in the boiler industry either due to high cost or inadequate capabilities for the application.
U.S. Pat. No. 5,072,616 to Sherrick (1991) cites an optical liquid level detecting system utilizing the differences between the optical refractive index for steam and water as the mechanism for determining the presence of water or steam in the path of an optical beam traversing a sensing region. Detection schemes operating upon this optical principle suffer from vulnerability to opaque scale buildup common to boilers that can impair the transmission or reception of the optical beam. Should this occur, the system will not reliably detect the presence or absence of liquid in the sensing region. Additionally, precise alignment of the beam detector is critical to the proper operation of the system.
U.S. Pat. No. 4,671,110 to de Kock (1987) cites an electronic means for liquid level detection in boilers. The patent describes a sensor utilizing the capacitative, conductive, or resistive properties of the liquid to detect liquid level. The described apparatus utilizes a custom-manufactured gauge glass subjected to a special surface etching procedure and subsequent vapor-deposition process to impart to the glass—either on the inside surface or the outside surface—a thin conductive coating such as stannous oxide to serve as an electrical conductor for liquid detection.
The requirement for a specially prepared gauge glass poses a significant practical limitation in terms of application due to the cost and general unavailability of a customized gauge glass, as compared to a standard gauge glass. Additionally, an exposed thin conductive coating on the outside surface of a gauge glass renders the sensor unstable and susceptible to spurious operations whenever the glass tube is approached or contacted by a person or other object. A gauge glass with a thin conductive coating on the inside surface of the glass is subject to degradation from scale build up or actual erosion of the coating off the surface of the glass by action of the process liquid and steam.